SYNOPSIS

DESCRIPTION

The class basic_istream defines member functions that assist
in reading and interpreting input from sequences controlled
by a stream buffer.
Two groups of member function signatures share common pro-
perties: the formatted input functions (or extractors) and
the unformatted input functions. Both groups of input func-
tions obtain (or extract) input characters from
basic_streambuf. They both begin by constructing an object
of class_basic_istream::sentry and, if this object is in
good state after construction, the function obtains the
requested input. The sentry object performs exception safe
initialization, such as controlling the status of the stream
or locking it in a multithread environment.
Some formatted input functions parse characters extracted
from the input sequence, converting the result to a value of
some scalar data type, and storing the converted value in an
object of that scalar type. The conversion behavior depends
directly on the locale object being imbued in the stream.

TYPES

char_type
The type char_type is a synonym for the template parame-
ter charT.
int_type
The type int_type is a synonym of type traits::in_type.
ios_type
The type ios_type is a synonym for basic_ios<charT,
traits> .
istream
The type istream is an instantiation of class
basic_istream on type char:
typedefbasic_istream<char> istream;
istream_type
The type istream_type is a synonym for
basic_istream<charT, traits>.
off_type
The type off_type is a synonym of type traits::off_type.
pos_type
The type pos_type is a synonym of type traits::pos_type.
streambuf_type
The type streambuf_type is a synonym for
basic_streambuf<charT, traits> .
traits_type
The type traits_type is a synonym for the template param-
eter traits.
wistream
The type wistream is an instantiation of class
basic_istream on type wchar_t:
typedefbasic_istream<wchar_t> wistream;

PUBLIC CONSTRUCTORS

PUBLIC DESTRUCTORS

virtual ~basic_istream();
Destroys an object of class basic_istream.

SENTRY CLASSES

explicit sentry(basic_istream<charT,traits>&,
bool noskipws=0);
Prepares for formatted or unformatted input. If the
basic_ios member function tie() is not a null pointer,
the function synchronizes the output sequence with any
associated stream. If noskipws is zero and the ios_base
member function flags() & skipws is nonzero, the function
extracts and discards each character as long as the next
available input character is a white space character. If
after any preparation is completed, the basic_ios member
function good() is true, the sentry conversion function
operator bool() returns true. Otherwise it returns false.
In a multithread environment the sentry object construc-
tor is responsible for locking the stream and the stream
buffer associated with the stream.
~sentry();
Destroys an object of class sentry. In a multithread
environment, the sentry object destructor is responsible
for unlocking the stream and the stream buffer associated
with the stream.
operator bool();
If after any preparation is completed, the basic_ios
member function good() is true, the sentry conversion
function operator bool() returns true else it returns
false.

EXTRACTORS

istream_type&
operator>>(istream_type&
(*pf) (istream_type&));
Calls pf(*this), then returns *this.
istream_type&
operator>>(ios_type& (*pf) (ios_type&));
Calls pf(*this), then returns *this.
istream_type&
operator>>(ios_base& (*pf) (ios_base&));
Calls pf(*this), then returns *this.
istream_type&
operator>>(bool& n);
Converts a Boolean value, if one is available, and stores
it in n. If the ios_base member function flag() &
ios_base::boolalpha is false, it tries to read an integer
value, which if found must be 0 or 1. If the boolalpha
flag is true, it reads characters until it determines
whether the characters read are correct according to the
locale function numpunct<>::truename() or
numpunct<>::falsename(). If no match is found, it calls
the basic_ios member function setstate(failbit), which
may throw ios_base::failure.
istream_type&
operator>>(short& n);
Converts a signedshort integer, if one is available, and
stores it in n, then returns *this.
istream_type&
operator>>(unsigned short& n);
Converts an unsignedshort integer, if one is available,
and stores it in n, then returns *this.
istream_type&
operator>>(int& n);
Converts a signedinteger, if one is available, and
stores it in n, then returns *this.
istream_type&
operator>>(unsigned int& n);
Converts an unsignedinteger, if one is available, and
stores it in n, then returns *this.
istream_type&
operator>>(long& n);
Converts a signedlong integer, if one is available, and
stores it in n, then returns *this.
istream_type&
operator>>(unsigned long& n);
Converts an unsignedlong integer, if one is available,
and stores it in n, then returns *this.
istream_type&
operator>>(float& f);
Converts a float, if one is available, and stores it in
f, then returns *this.
istream_type&
operator>>(double& f);
Converts a double, if one is available, and stores it in
f, then returns *this.
istream_type&
operator>>(long double& f);
Converts a longdouble, if one is available, and stores
it in f, then returns *this.
istream_type&
operator>>(void*& p);
Extracts a void pointer, if one is available, and stores
it in p, then returns *this.
istream_type&
operator>>(streambuf_type* sb);
If sb is null, calls the basic_ios member function
setstate(badbit), which may throw ios_base::failure. Oth-
erwise extracts characters from *this and inserts them in
the output sequence controlled by sb. Characters are
extracted and inserted until any of the following occurs:
- An end-of-file on the input sequence
- A failure when inserting in the output sequence
- An exception
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. If failure was due to catching
an exception thrown while extracting characters from sb
and failbit is on in exception(), then the caught excep-
tion is rethrown.
istream_type&
operator>>(streambuf_type& sb);
Extracts characters from *this and inserts them in the
output sequence controlled by sb. Characters are
extracted and inserted until any of the following occurs:
- An end-of-file on the input sequence
- A failure when inserting in the output sequence
- An exception
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. If failure was due to catching
an exception thrown while extracting characters from sb
and failbit is on in exception(), then the caught excep-
tion is rethrown.

UNFORMATTED FUNCTIONS

streamsize
gcount() const;
Returns the number of characters extracted by the last
unformatted input member function called.
int_type
get();
Extracts a character, if one is available. Otherwise, the
function calls the basic_ios member function
setstate(failbit), which may throw ios_base::failure.
Returns the character extracted or returns traits::eof(),
if none is available.
istream_type&
get(char_type& c);
Extracts a character, if one is available, and assigns it
to c. Otherwise, the function calls the basic_ios member
function setstate(failbit), which may throw
ios_base::failure.
istream_type&
get(char_type* s, streamsize n,char_type delim);
Extracts characters and stores them into successive loca-
tions of an array whose first element is designated by s.
Characters are extracted and stored until any of the fol-
lowing occurs:
- n-1 characters are stored
- An end-of-file on the input sequence
- The next available input character == delim.
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. In any case, it stores a null
character into the next successive location of the array.
istream_type&
get(char_type* s, streamsize n);
Calls get(s,n,widen("\n")).
istream_type&
get(streambuf_type& sb,char_type delim);
Extracts characters and inserts them in the output
sequence controlled by sb. Characters are extracted and
inserted until any of the following occurs:
- An end-of-file on the input sequence
- A failure when inserting in the output sequence
- The next available input character == delim.
- An exception
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. If failure was due to catching
an exception thrown while extracting characters from sb
and failbit is on in exception(), then the caught excep-
tion is rethrown.
istream_type&
get(streambuf_type& sb);
Calls get(sb,widen("\n")).
istream_type&
getline(char_type* s, streamsize n, char_type delim);
Extracts characters and stores them into successive loca-
tions of an array whose first element is designated by s.
Characters are extracted and stored until any of the fol-
lowing occurs:
- n-1 characters are stored
- An end-of-file on the input sequence
- The next available input character == delim.
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. In any case, it stores a null
character into the next successive location of the array.
istream_type&
getline(char_type* s, streamsize n);
Calls getline(s,n,widen("\n")).
istream_type&
ignore(streamsize n=1, int_type delim=traits::eof());
Extracts characters and discards them. Characters are
extracted until any of the following occurs:
- n characters are extracted
- An end-of-file on the input sequence
- The next available input character == delim.
int_type
peek();
Returns traits::eof() if the basic_ios member function
good() returns false. Otherwise, returns the next avail-
able character. Does not increment the current get
pointer.
istream_type&
putback(char_type c);
Inserts c in the putback sequence.
istream_type&
read(char_type* s, streamsize n);
Extracts characters and stores them into successive loca-
tions of an array whose first element is designated by s.
Characters are extracted and stored until any of the fol-
lowing occurs:
- n characters are stored
- An end-of-file on the input sequence
If the function does not store n characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure.
streamsize
readsome(char_type* s, streamsize n);
Extracts characters and stores them into successive loca-
tions of an array whose first element is designated by s.
If rdbuf()->in_avail() == -1, calls the basic_ios member
function setstate(eofbit).
- If rdbuf()->in_avail() == 0, extracts no char-
acters
- If rdbuf()->in_avail() > 0, extracts <br>min(
rdbuf()->in_avail(), n)
In any case the function returns the number of characters
extracted.
istream_type&
seekg(pos_type& pos);
If the basic_ios member function fail(), returns false,
executes rdbuf()->pubseekpos(pos), which positions the
current pointer of the input sequence at the position
designated by pos.
istream_type&
seekg(off_type& off, ios_base::seekdir dir);
If the basic_ios member function fail() returns false,
executes rdbuf()->pubseekpos(off,dir), which positions
the current pointer of the input sequence at the position
designated by off and dir.
int
sync();
If rdbuf() is a null pointer, return -1. Otherwise, calls
rdbuf()->pubsync() and if that function returns -1 calls
the basic_ios member function setstate(badbit). The pur-
pose of this function is to synchronize the internal
input buffer, with the external sequence of characters.
pos_type
tellg();
If the basic_ios member function fail() returns true,
tellg() returns pos_type(off_type(-1)) to indicate
failure. Otherwise it returns the current position of the
input sequence by calling rdbuf()->pubseekoff(0,cur,in).
istream_type&
unget();
If rdbuf() is not null, calls rdbuf()->sungetc(). If
rdbuf() is null or if sungetc() returns traits::eof(),
calls the basic_ios member function setstate(badbit).

NON-MEMBER FUNCTIONS

template<class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, charT& c);
Extracts a character if one is available, and stores it
in c. Otherwise the function calls the basic_ios member
function setstate(failbit), which may throw
ios_base::failure.
template<class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, charT* s);
Extracts characters and stores them into successive loca-
tions of an array whose first element is designated by s.
If the ios_base member function is.width() is greater
than zero, then is.width() is the maximum number of char-
acters stored. Characters are extracted and stored until
any of the following occurs:
- If is.width()>0, is.width()-1 characters are
extracted
- An end-of-file on the input sequence
- The next available input character is a white
space
If the function stores no characters, it calls the
basic_ios member function setstate(failbit), which may
throw ios_base::failure. In any case, it then stores a
null character into the next successive location of the
array and calls width(0).
template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is,
unsigned char& c);
Returns is >> (char&)c.
template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is,
signed char& c);
Returns is >> (char&)c.
template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is,
unsigned char* c);
Returns is >> (char*)c.
template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is,
signed char* c);
Returns is >> (char*)c.
template<class charT, class traits>
basic_istream<charT, traits>&
ws(basic_istream<charT, traits>& is);
Skips any white space in the input sequence and returns
is.

EXAMPLE

//
// stdlib/examples/manual/istream1.cpp
//
#include<iostream>
#include<istream>
#include<fstream>
void main ( )
{
using namespace std;
float f= 3.14159;
int i= 3;
char s[200];
// open a file for read and write operations
ofstream out("example", ios_base::in | ios_base::out
| ios_base::trunc);
// tie the istream object to the ofstream filebuf
istream in (out.rdbuf());
// output to the file
out << "Annie is the Queen of porting" << endl;
out << f << endl;
out << i << endl;
// seek to the beginning of the file
in.seekg(0);
f = i = 0;
// read from the file using formatted functions
in >> s >> f >> i;
// seek to the beginning of the file
in.seekg(0,ios_base::beg);
// output the all file to the standard output
cout << in.rdbuf();
// seek to the beginning of the file
in.seekg(0);
// read the first line in the file
// "Annie is the Queen of porting"
in.getline(s,100);
cout << s << endl;
// read the second line in the file
// 3.14159
in.getline(s,100);
cout << s << endl;
// seek to the beginning of the file
in.seekg(0);
// read the first line in the file
// "Annie is the Queen of porting"
in.get(s,100);
// remove the newline character
in.ignore();
cout << s << endl;
// read the second line in the file
// 3.14159
in.get(s,100);
cout << s << endl;
// remove the newline character
in.ignore();
// store the current file position
istream::pos_type position = in.tellg();
out << "replace the int" << endl;
// move back to the previous saved position
in.seekg(position);
// output the remain of the file
// "replace the int"
// this is equivalent to
// cout << in.rdbuf();
while( !char_traits<char>::eq_int_type(in.peek(),
char_traits<char>::eof()) )
cout << char_traits<char>::to_char_type(in.get());
cout << "\n\n\n" << flush;
}
//
// istream example #2
//
#include <iostream>
void main ( )
{
using namespace std;
char p[50];
// remove all the white spaces
cin >> ws;
// read characters from stdin until a newline
// or 49 characters have been read
cin.getline(p,50);
// output the result to stdout
cout << p;
}